I don't know how often Trend Micro updates the manual updates a day. But if they only release 1 manual update package a day compared with automatic updates. That could be the problem that the manual package you have downloaded is a little bit out dated. It could also be that when you are using automatic updates Trend Micro writes to a file and this doesn't happen when you only extract the signatures. If the virus pattern matches the one off your zip file than everything is fine.